It is desirable to degrade digital signals in many situations so as to restrict access. For instance, pay-TV broadcasts are degraded so those who haven't paid for the program cannot watch it because the picture is unclear, while those who have paid for the program see a clear picture because their recovery apparatus has been enabled. Most recently, as a result of the digital audio revolution, it is desirable to restrict MP3 (a standard bit-rate compressed audio file format) access. It is also desirable to produce inexpensive portable MP3 players, which in turn require that recovery of the original signal be simple.
There are numerous existing methods of degrading digital content, a.k.a. scrambling. Some methods require a key to de-scramble the content, whereas others do not. Most scrambling or degrading methods are based upon either adding an interference signal to the digital content, or moving the bits around. Other methods use encryption, but this is very computationally intense.